A fully formed egg may lodge in the shell gland or vagina because the egg is too big (eg, double-yolked) or because of hypocalcemia, calcium tetany, or previous trauma (usually pecking) to the vent and/or vagina that obstructs oviposition. This condition may be more prevalent in young hens that are brought into production before body development is adequate or in hens that are overweight or obese. It occurs more often during spring and summer months because of overstimulation of birds by increasing light intensity and day length, which can be compounded by rapid increases in the amount of feed and/or excessive protein (lysine). This is a medical emergency in pet birds but is usually recognized only during necropsy in commercial poultry. When impaction occurs, eggs that continue to form create layers of albumen and yolk material, and the oviduct becomes very large. Some eggs are refluxed to the abdominal cavity (abdominal laying), and affected hens assume a penguin-like posture.

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Influenza A viruses are divided into 16 hemagglutinin (H1-16) and 9 neuraminidase (N1-9) subtypes. Which of the following subtypes is most likely to cause a highly pathogenic form of avian influenza in poultry?

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